The present invention relates to a filter bank for use in digital signal processing, and in particular, to an efficient filter bank structure that uses oversampling and separate odd and even subband processing paths to reduce aliasing between subbands.
The digital filter bank is an enabling technology for modern audio and video processing systems, and, more recently, digital data communication systems such as Orthogonal Frequency-Division Multiplexing (OFDM). FIG. 1 illustrates a typical conventional filter bank structure. A circuit 100 processes an input digital signal x(n) in N separate paths. An analysis filter bank 120 includes respective analysis filters that decompose and transform the input signal x(n) into its frequency-domain subband components xb(0), . . . ,xb(N−1) according to respective transfer functions H(0), H(1) . . . H(i), . . . H(N−1). The subband components can be processed by distinct subband processors SB(0), SB(1) . . . SB(i), . . . , SB(N−1) of a subband processor 140 to be transformed into respective components yb(0), yb(1) . . . , yb(i), . . . , yb(N−1). Various types of subband processing can be performed.
A synthesis filter bank 160 reassembles and transforms the processed components into an output signal y(n) using synthesis filters F(0), F(1), F(i), . . . F(N−1).
Many text books provide a good introduction to the theory of digital filter banks.
Of particular interest is the class of critically-sampled uniform filter banks, which have found wide application in the areas of audio and video processing. For example, the DCT (Discrete Cosine Transform) is used in the MPEG-2 video compression engine, whereas TDAC (Time Domain Aliasing Cancellation) and Cosine-modulated filter banks have been standardized into the MPEG-2 audio compression algorithms and Dolby Lab's™ AC3 compression algorithm.
While the efficiency of these filter banks makes them suitable for many signal processing applications, they suffer from aliasing between the subbands. Most practical filters have finite rejection at the Nyquist frequency so the signals beyond the Nyquist frequency are not sufficiently attenuated prior to downsampling, and appear as aliased components in the downsampled signal. Most of these filter banks are designed to be “aliasing canceling”, which means that the synthesis filter bank is specifically designed to account for, and cancel, the aliasing caused by the analysis filter banks. However, this cancellation property severely constrains the kind of processing that can be introduced by the subband processors SB(0), . . . , SB(N−1). For example, a gain factor applied to one of the subband processors SB(i) will cause aliasing in the neighboring subbands, SB(i−1) and SB(i+1), during synthesis. This makes the use of such filter banks unsuitable for applications such as subband equalizers or noise shaper.
To reduce aliasing, a higher order filter can be used. However, this decreases the temporal resolution of the filter banks as well as the computational efficiency since additional calculations are required.
Another approach is to try to avoid the aliasing in the first place, by oversampling the subband components of interest. However, aliasing is still present between the subbands unless the oversampling ratio approaches the number of subbands M. This approach also is not computationally efficient.
Accordingly, it would be desirable to provide a filter bank structure for subband processing that avoids constraining the type of subband processing, is computationally efficient, avoids aliasing between subbands, can be implemented using fast filter banks, and which has a relatively low filter order, and good temporal response and stop band rejection.
The filter bank structure should be suitable for use with any type of digital input signal, including one-dimensional signals such as audio signals, and two-dimensional signals such as video signals, and should allow any type of subband processing, including embedding of auxiliary data, noise shaping, and equalizing.
The filter bank structure should allow the use of real or complex filter banks.
The present invention provides a filter structure having the above and other advantages.